A disk image is a computer file containing the complete contents and structure of a data storage device. The term has been generalized to cover any such file, whether taken from an actual physical storage device or not.

An ordinary backup only backs up the files it can access; booting information, and files locked by an operating system or being changed at the time, may not be saved. A disk image contains all these, and faithfully replicates the data, so it is commonly used for backing up disks with operating systems, or bootable CD/DVDs.

The image is saved as a file, which for a full CD or system disk may be quite large (~10 Megabyte to several Gigabyte). This file can be saved onto a hard drive, CD, DVD or other media, for later use.

Disk images can either be compressed using some type of compression algorithm like LZW, or uncompressed ( raw ). Images of CD-ROMs most often carry the file name extension .ISO image, referring to the ISO 9660 file system commonly used on such discs. The .iso format is the most common format for Linux distribution and other online images. Other common CD image formats are .nrg (Nero Burning ROM s proprietary format), and .bin/.cue.

= CD Sector Size FAQ =

# A standard 74 min CD is made by 333,000 sectors. # Each sector is 2352 bytes, and contains 2048 bytes of PC (MODE1) Data, 2336 bytes of PSX/VCD (MODE2) Data or 2352 bytes of AUDIO. # The difference between sector size and data content are the Headers info and the Error Correction Codes, that are big for Data (high precision required), small for VCD (standard for video) and none for audio. # If you extract data in RAW format (standard for creating images) you always extract 2352 bytes per sector, not 2048/2336/2352 bytes depending on data type (basically, you extract the whole sector). This fact has two main consequences: ## You can record data at very high speed (40x) without losing information, but if you try to do the same with PSX or Audio you get unredable CD (for PSX) or audio CD with lots of clicks because there are no error correction codes (and error are more likely to occur if you record at high speed.) ## On a 74 min CD you can fit very large RAW images,up to 333,000 x 2352 = 783,216,000 bytes (747 Mb). This should be the upper limit for a RAW image created from a 74 min CD. Remember that if you store standard data (backup files), you can burn only 333,000 x 2048 = 681,984,000 bytes (the well known 650 MB limit). # Please note that an image size is ALWAYS a multiple of 2352 bytes (you extract SECTORS), if extracted in RAW mode.

= .CUE/.BIN =

The .cue / .bin format developed by Jeff Arnold for CDRWIN can encode CD Image formats in either 2048 or 2324 bytes per sector. The BIN file is a binary copy of an entire CD/DVD disc. The BIN file contains all the data stored on the original disc, not only its files and folders but also its system-specific information such as, booting, volume_(computing) attributes and any other data. BIN files are usually bigger than .iso files because in contrast to .iso files they are not a bit for bit copy of the entire CD/DVD.

.bin files (or .RAW files) are images extracted in pure RAW format. That is 2352 bytes sectors, the full CD sector content: user data, sector header, error correction codes (ECC) and error detection codes (EDC). Once again, each sector is converted to digital data in the .BIN file, but more stuff is copied and the resulting file will be bigger. The .BIN file should be 251,000 x 2352 = 590,352,000 bytes big. This process will copy ANYTHING on the disc, so it is useful for exotic discs (multiple tracks, mixed track type Audio+Data or Data+Audio) and for non-PC CDs (PSX, VCD, MAC).

The .cue file is a datasheet that describes the data stored in the .bin file. The .cue file is in fact a plain text file. A typical .cue file is as follows:

FILE IMAGE.BIN BINARY TRACK 01 MODE1/2352 INDEX 01 00:00:00

The file would be saved as IMAGE.CUE

= .ISO =

The .ISO file is a digital copy of CD contents made this way: the ripper searches for the sectors of the CD that have been used, say 251,000 for instance (there are 330,000 sectors on a 74 min CD and 360,000 sectors on a 80 min CD). Each sector is copied on the .ISO file, one by one, and only 2048 bytes for each sector (only the ones containing the user data) are copied. The .ISO file should then be of size 251,000 x 2048 = 514,408,000 bytes. (It will be slightly bigger if the extractor puts a header on the file, like Nero .NRG files that are .ISO files plus a small file header)

=Apple Disk Image=

For Apple Macintosh computers, disk images have been available as a feature of the operating system before System 7 (Macintosh). Mounting a compatible disk image is achieved by Double-click (computing) (opening) the image file. The archived volume is then available as a normal disk volume, indistinguishable at first glance from the physical volume. Modern Apple Disk Images for Mac OS X typically end in .dmg . Using NetBoot, client computers can start up over a network from a server-based disk image that contains system software.

= Usage =

A common use of disk images is for remote distribution of software such as , creating an image of a software package and subsequently distributing it.

Another common use is to provide virtual disk drive space to be used by emulators and virtual machines. This can prevent the CD from getting damaged. It can also reduce bulk when one wishes to carry the contents of the CD along with oneself: one can store disk images to a relatively lightweight storage device which has a higher storage capacity than that of a CD.

= See also =

= References =